Integrated method for synthesis propylene oxide

ABSTRACT

The present invention relates to an integrated process for the synthesis of propylene oxide, which comprises at least the following steps: 
         (i) dehydrogenation of propane to give a substream T ( 0 ) comprising at least propane, propene and hydrogen;    (ii) fractionation of the substream T ( 0 ) to give at least one gaseous hydrogen-rich substream T ( 2 ) and a substream T ( 1 ) comprising at least propene and propane;    (iii) synthesis of hydrogen peroxide using the substream T ( 2 ), giving a substream T ( 4 ) which is rich in hydrogen peroxide and a gaseous substream T ( 6 );    (iv) fractionation of the substream T ( 1 ) to give at least one propane-rich substream T ( 5 ) and at least one propene-rich substream T ( 3 );    (v) reaction of the at least one substream T ( 3 ) with substream T ( 4 ) to give propylene oxide.

The present invention relates to an integrated process for the synthesisof propylene oxide.

In the process of the present invention, the starting materials for thepropylene synthesis are prepared via at least the steps propanedehydrogenation and direct synthesis of hydrogen peroxide and reacted togive propylene oxide.

The product propylene oxide forms the basis of a wide variety ofchemical processes. The worldwide production capacity was about 2.9million metric tons per annum in 1985, about 4.0 million metric tons perannum in 1993 and has grown continually since. Propylene oxide hashitherto been prepared predominantly by the chlorohydrin process and viaindirect oxidation processes using hydroperoxides. The greatdisadvantages of these two processes are, for example, the wastewaterproblem and the by-product problem in the chlorohydrin process and theproduction of large amounts of oxygen-containing coproducts in a processvia the indirect oxidation. These problems have in recent years led todevelopment of alternative processes in the area of propylene oxidesynthesis.

For example, close integration of chlorine production into the propyleneoxide synthesis via the chlorohydrin process has enabled the economicsto be improved.

In the propylene oxide synthesis by indirect oxidation methods, too, thedisadvantageously large amount of coproducts has been able to be reducedin the propene oxidation, for example by use of percarboxylic acidswhich are prepared by means of hydrogen peroxide in a step preceding thepropene oxidation, which improved the economic viability of thisprocess.

Further advantageous developments in the field of propylene oxidesynthesis are discussed, inter alia, in the documents DE 101 37 543.3,DE 101 35 296.4, DE 101 05 527.7 and DE 100 32 885.7.

Nevertheless, owing to the wide-ranging uses of propylene oxide, forexample in the polymerization of alcohols, and the associated continualdemand, there continues to be a need for a propylene oxide productionprocess which makes it possible for the overall synthesis of propyleneoxide, i.e. starting from the preparation of the starting materialsthrough to recycling of the products other than propylene oxideobtained, to be made more economical and thus more competitive.

It is an object of the present invention to provide a further processfor the integrated synthesis of propylene oxide.

We have found that this object is achieved by an integrated process forthe synthesis of propylene oxide, which comprises at least the followingsteps:

-   -   (i) dehydrogenation of propane to give a substream T (0)        comprising at least propane, propene and hydrogen;    -   (ii) fractionation of the substream T (0) to give at least one        gaseous hydrogen-rich substream T (2) and a substream T (1)        comprising at least propene and propane;    -   (iii) synthesis of hydrogen peroxide using the substream T (2),        giving a substream T (4) which is rich in hydrogen peroxide and        a gaseous substream T (6);    -   (iv) fractionation of the substream T (1) to give at least one        propane-rich substream T (5) and at least one propene-rich        substream T (3);    -   (v) reaction of the at least one substream T (3) with substream        T (4) to give propylene oxide.

A preferred embodiment of the process, i.e. the essential steps makingup the process, is shown schematically in FIG. 1 (FIG. 1).

The present invention further provides an extended integrated processfor the synthesis of propylene oxide, which comprises at least thefollowing steps:

-   -   (a) dehydrogenation of propane to give a substream T (0)        comprising at least propane, propene and hydrogen;    -   (b) fractionation of the substream T (0) to give at least one        gaseous hydrogen-rich substream T (2) and a substream T (1)        comprising at least propene and propane;    -   (c) fractionation of the substream T (1) to give at least one        propane-rich substream T (5) and at least one propene-rich        substream T (3);    -   (d) separation of the substream T (5) into at least the        substreams T (5 a) and T (5 b);    -   (e) synthesis of hydrogen peroxide using the substream T (2)        which is combined with at least the substream T (5 a), giving a        substream T (4) which is rich in hydrogen peroxide and a gaseous        substream T (6 a);    -   (f) recirculation of the substream T (6 a) to step (a);    -   (g) reaction of the at least one substream T (3) with substream        T (4) to give propylene oxide.

A preferred embodiment of the extended integrated process for thesynthesis of propylene oxide, i.e. the essential steps making up theprocess, is shown schematically in FIG. 2 (FIG. 2).

Step (i) or step (a) of the process of the present invention comprisesdehydrogenation of propane to give a substream T (0) comprising at leastpropane, propene and hydrogen.

The dehydrogenation of propane in the process of the present inventioncan in principle be carried out by all methods known to those skilled inthe art for the dehydrogenation of propane, for example steam crackingor catalytic cracking and also, in particular, catalytic dehydrogenationin the presence or absence of oxygen or oxygen-containing mixtures.

To achieve economically viable conversions, based on a single pass, inthe dehydrogenation of propane, it is necessary to work at relativelyhigh reaction temperatures. These are generally from 300 to 700° C.

Since the dehydrogenation, i.e. the cleavage of a C—H bond, is generallyless favored kinetically than cracking, i.e. the cleavage of a C—C bond,it is preferably carried out over catalysts which are selective fordehydrogenation. These are usually of such a nature that they give agood yield of dehydrogenation products in the absence of oxygen in theabovementioned temperature range. At a space velocity of propane overthe catalysts of, for example, 1000 h⁻¹ (i.e. standard liters of propaneper liter of catalyst and hour), the yield of propylene is generally atleast 30 mol % based on the propane used in a single pass. By-productssuch as methane, ethylene and ethane are formed in only subordinateamounts.

Since dehydrogenation of propane proceeds with an increase in volume,the conversion can in principle be increased by reducing the partialpressure of the products. This can be achieved in a simple manner by,for example, carrying out the dehydrogenation under reduced pressureand/or with introduction of essentially inert diluent gases. For thepurposes of the present invention, steam is such a preferred inertdiluent gas. Further diluent gases suitable for the dehydrogenation ofpropane are, for example, CO₂, N₂ and noble gases such as He, Ne and Ar.

Dilution with steam generally gives the further advantage of reducedcarbonization of the catalysts used, since the steam reacts with anycarbon deposits being formed according to the principle of coalgasification. Furthermore, steam can easily be separated partly orcompletely from the product mixture. Accordingly, when steam is used asdiluent in the propane dehydrogenation in the process of the presentinvention, it can be separated off from the product stream T (0) by, forexample, condensation.

In the dehydrogenation of propane in step (i) or step (a), it is inprinciple possible to use all dehydrogenation catalysts known to thoseskilled in the art for this purpose. For example, catalysts which areoxidic in nature and comprise chromium oxide and/or aluminum oxide orcatalysts comprising at least one substantially noble metal, e.g.platinum, deposited on at least one generally oxidic support can beused.

The dehydrogenation catalysts described in the following documents can,inter alia, be used for the purposes of the present invention: WO99/46039, U.S. Pat. No. 4,788,371, EP-A 705 136, WO 99/29420, U.S. Pat.No. 5,220,901, U.S. Pat. No. 5,430,220, U.S. Pat. No. 5,877,469, EP-A117 146, DE 199 37 106, DE 199 37 105, and DE 199 37 107.

In particular, the dehydrogenation catalysts described in the examplesof DE 199 37 107 can be used. These are dehydrogenation catalystscomprising from 10 to 99.9% by weight of zirconium dioxide, from 0 to60% by weight of aluminum oxide, silicon dioxide and/or titanium dioxideand from 0.1 to 10% by weight of at least one element of the first orsecond main group, the third or eighth transition group of the PeriodicTable of the Elements, lanthanum and/or tin, with the proviso that thesum of the percentages by weight is 100.

To carry out the dehydrogenation of propane in the process of thepresent invention, it is in principle possible to use all reactor typesand process variants known to those skilled in the art for this purpose,for example those which are described in the documents cited in theprevious section in respect of the dehydrogenation catalysts.

For example, the propane used in the dehydrogenation of propane can beoxidized homogeneously to propene in the presence of molecular oxygen atelevated temperature as described in U.S. Pat. No. 3,798,283. For thepurposes of the invention, the oxygen source can be either pure oxygenor a mixture of oxygen and inert gas.

The heterogeneously catalyzed oxydehydrogenation of propane to propenedescribed in DE 195 30 45 can also be employed for the propanedehydrogenation. Here, the propane is converted into propene in thepresence of air or an oxygen-containing mixture in a fixed-bed orfluidized-bed reactor containing catalyst.

For the purposes of the invention, it is also possible to convertpropane into propene by homogeneously and/or heterogeneously catalyzedoxydehydrogenation by means of molecular oxygen in a manner analogous tothe process described in DE 198 37 517.

In principle, all oxydehydrogenation processes which can be used for thepurposes of the invention can be carried out in at least one reactionvessel containing catalytically active substance, for example afixed-bed reactor or a fluidized-bed reactor. In this reactor, thepropane is converted into propene over the catalytically activesubstance used in each case via reaction steps known to those skilled inthe art.

A further possibility for the dehydrogenation of propane in the processof the present invention is the Oleflex™ process or methods similarthereto. In this process, the feed propane, admixed with pure orrecycled hydrogen, is converted into propene in at least one reactorcomprising at least one suitable catalyst bed.

In principle, propane can be partly or virtually completelydehydrogenated to propene in the presence of a dehydrogenation catalyst.Partial dehydrogenation forms a product gas mixture comprising unreactedpropane and the propene formed together with secondary constituents suchas hydrogen, water, further cracking products of propane, CO and CO₂.The dehydrogenation of propane can be carried out with or without anoxygen-containing gas as cofeed.

The partial heterogeneously catalyzed dehydrogenation of propanegenerally proceeds endothermically, i.e. the heat/energy required to setthe reaction temperature required is introduced into the reaction gasbefore and/or during the catalytic dehydrogenation.

Owing to the relatively high reaction temperature required for thepropane dehydrogenation, formation of small amounts of high-boiling highmolecular weight organic compounds, sometimes even carbon, can occur andthese may deposit on the catalyst surface. To minimize or avoid thisunfavorable accompanying phenomenon, the starting material propane canbe diluted with hydrogen in the process of the present invention, sothat any carbon formed can be largely eliminated according to theprinciple of coal hydrogenation.

A comprehensive description of reactor types and modes of operationwhich are suitable in principle for the dehydrogenation of propane inthe process of the present invention is given in “Catalytica® StudiesDivision, Oxydative Dehydrogenation and Alternative DehydrogenationProcesses, Study No. 4192 OD, 1993, 430, Ferguson Drive, Mountain View,Calif., 94043-5272, U.S.A.”.

A suitable form of reactor for step (i) or step (a) of the process ofthe present invention is a fixed-bed tube reactor or shell-and-tubereactor. In this, the catalyst is present as a fixed bed in a reactiontube or in a bundle of reaction tubes. The dehydrogenation of propanecan be carried out in the absence of oxygen or, when suitable catalystformulations are employed, with introduction of oxygen as cofeed. Thereaction tubes can be heated by a gas, for example a hydrocarbon such asmethane, being burnt in the space surrounding the reaction tubes.

In a preferred embodiment of the integrated process for the synthesis ofpropylene oxide, all or some of the gaseous substream T (6) from step(iii) can be burnt to produce energy and the energy can be used in thepropane dehydrogenation, for example for indirect heating of thereaction tubes in question.

It is advantageous to apply this indirect form of heating only to thefirst about 20-30% of the length of the fixed catalyst bed and to heatthe remaining length of the bed to the required reaction temperature bymeans of the radiant heat liberated in the indirect heating.

Since the catalyst is generally, depending on the space velocity of thegas and the conversion, deactivated more or less rapidly bycarbonization, it is advantageous to generate it at regular intervals.The substream T (6) from step (iii) can be used for regeneration in theprocess of the present invention. Further regeneration methods which canbe used according to the present invention are described in WO 98/55430and the prior art cited therein.

In a further embodiment of the present invention, the dehydrogenation ofpropane can be carried out in a moving bed reactor. The moving catalystbed can be accommodated in, for example, a radial flow reactor. In this,the catalyst slowly moves from the top downward, while the reaction gasmixture flows radially. Since the reactors in this process are operatedpseudoadiabatically it is advantageous to employ a plurality of reactorsconnected in series.

The mixture entering each reactor can be heated to the required reactiontemperature upstream of the reactor by indirect heating. It is likewisepossible to heat the gas mixture entering each reactor to the requiredreaction temperature by combustion of hydrogen in the presence of addedoxygen (autothermal operation). Both in the integrated process and theextended integrated process for the synthesis of propylene oxideaccording to the present invention, it is advantageous to use all orsome of the substream T (6) or T (6 a) from step (iii) for heating theinflowing gas mixture or for aiding autothermal operation of the propanedehydrogenation of step (a).

Furthermore, the use of a plurality of reactors enables largedifferences in the temperature of the reaction gas mixture betweenreactor inlet and reactor outlet to be avoided and high totalconversions to be achieved. When the catalyst bed has left the movingbed reactor, it is passed to regeneration and subsequently reused. Theregeneration of the catalyst in a moving bed reactor is generallycarried out continuously. Here, all or part of the exhausted catalyst isdischarged at the end of a reactor, in particular at the end of the lastreactor, and passed to at least one subsequent regeneration step. Afterregeneration, the catalyst is returned to the beginning of a reactor, inparticular the beginning of the first reactor.

The heterogeneously catalyzed dehydrogenation of propane carried out ina fluidized bed, the operation of which is described in “Chem. Eng. Sci.1992 b 47 (9-11), 2313”, is likewise a possible way of carrying out thedehydrogenation of propane for the purposes of the present invention. Inthis process, the propane does not have to be diluted. It isadvantageous to operate two fluidized beds side by side in thedehydrogenation, so that one of them can generally be in the state ofregeneration. The regeneration of the fluidized beds can, in a preferredembodiment of the integrated process for the synthesis of propyleneoxide, be carried out using the substream T (6) from step (iii). Theheat required for the dehydrogenation is introduced into the reactionsystem by the dehydrogenation catalyst being preheated to the reactiontemperature. In a preferred embodiment of the present invention,preheating can also be carried out by means of the energy produced byburning all or some of the substream T (6) from step (iii).

In a further embodiment of the present invention, the introduction of anoxygen-containing cofeed makes it possible to dispense with thepreheater or intermediate heaters or the indirect heating via thereactor surfaces and to generate all or part of the necessary heatdirectly in the reactor system by combustion of hydrogen and/orhydrocarbons in the presence of molecular oxygen. In such an embodiment,the substream T (6 a) from step (e) of the novel extended integratedprocess for the synthesis of propylene oxide is, in a further step (f),recirculated to the propane dehydrogenation step (a), thus makinglargely autothermal operation of the propane dehydrogenation possible.If appropriate, a hydrogen-containing cofeed can be additionally mixedin.

Carrying out the dehydrogenation of propane in a tray reactor is anotherpossibility for the purposes of the present invention. A tray reactorcomprises one or more successive catalyst beds through which thereaction gas preferably flows radially or axially. In general, such atray reactor is operated using a fixed catalyst bed. In the simplestcase, the fixed catalyst beds are arranged axially in a shaft furnacereactor or in the annular gaps of concentric mesh cylinders. One shaftfurnace reactor corresponds to one tray. In a procedure without oxygenas cofeed, the reaction gas mixture is subjected to intermediate heatingon its way from one catalyst bed to the next catalyst bed in the trayreactor, for example by passing it over heat exchanger surfaces heatedby means of hot gases or by passing it through tubes heated by means ofhot combustion gases.

In a procedure using oxygen, a limited amount, depending on thedehydrogenation catalyst used, of the hydrocarbons present in thereaction gas, possibly also carbon deposited on the catalyst surfaceand/or hydrogen formed during the propane dehydrogenation and/or addedto the reaction gas is burnt. The heat of reaction liberated in this wayalso makes it possible to operate the propane dehydrogenationautothermally.

It is also possible, in a further embodiment of the process of thepresent invention, to carry out step (a), viz. the dehydrogenation ofpropane, autothermally. For this purpose, an oxygen-containing gas isadditionally mixed into the reaction mixture of the propanedehydrogenation in at least one reaction zone and the hydrogen presentin the reaction gas mixture is burnt, thus generating at least part ofthe necessary heat of dehydrogenation directly in the reaction mixturein the reaction zone or zones.

The dehydrogenation of propane is preferably carried out in thecirculation mode described in DE 102 11 275.4.

To operate the propane dehydrogenation step (a) autothermally in thenovel extended integrated process for the synthesis of propylene oxide,all or some of the gaseous, hydrogen-containing substream T (6 a)obtained in step (e) is recirculated to step (a) and burnt.

Regulating the amount of oxygen added via substream T (6 a) makes itpossible to control the reaction temperature in step (a). At the sametime, the selectivity of the propane dehydrogenation in step (a) can becontrolled by regulating the amount of hydrogen added via substream T (6a).

The quantity of heat provided for the dehydrogenation of propane topropene, which is generated by combustion of the hydrogen present in thereaction gas mixture and possibly hydrocarbons present in the reactiongas mixture and/or carbon present in the form of carbon deposits, isregulated via the amount of oxygen-containing gas added to the reactiongas mixture.

Additional oxygen introduced can be fed in either as molecular oxygen oras oxygen-containing gas, e.g. in admixture with inert gases.

In a preferred embodiment of the invention, additionally introducedmolecular oxygen is employed for this purpose.

In a further preferred embodiment of the present invention, theoxygen-containing gas in question is the substream T (6 a) from thehydrogen peroxide synthesis in step (e).

The inert gases and the resulting combustion gases generally have anadditional diluent effect and thus promote the heterogeneously catalyzeddehydrogenation. The hydrogen burnt to generate heat can be the hydrogenformed in the hydrocarbon dehydrogenation or additional hydrogen addedto the reaction gas mixture, for example via the hydrogen-containingsubstream T (6 a) from step (e) in the novel extended integrated processfor the synthesis of propylene oxide.

The amount of hydrogen added is basically such that the molar ratio ofH₂/O₂ in the reaction gas mixture directly after the feed point is from0 to 10 mol/mol. This applies both in the case of multistage reactorsand for the intermediate introduction of hydrogen and oxygen. Thehydrogen combustion in question occurs catalytically, with thedehydrogenation catalyst used generally also catalyzing the combustionof the hydrocarbons and the combustion of hydrogen in the presence ofoxygen, so that in principle no specific oxidation catalyst differentfrom this is necessary. However, it is of course also possible to employone or more oxidation catalysts. These selectively catalyze thecombustion of hydrogen to oxygen in the presence of hydrocarbons. As aresult, the combustion of hydrocarbons with oxygen to form CO, CO₂ andH₂O occurs to only a subordinate extent: which has a significantpositive effect on the achieved selectivity of the formation of propene.The dehydrogenation catalyst and the oxidation catalyst are preferablypresent in different reaction zones.

In the case of a multistage reaction, the oxidation catalyst can bepresent in only one reaction zone, in a plurality of reaction zones orin all reaction zones.

The catalyst which selectively catalyzes the oxidation of hydrogen inthe presence of hydrocarbons is preferably located in places at whichthe oxygen partial pressures are higher than at other places in thereactor, in particular in the vicinity of the feed point for theoxygen-containing gas. Oxygen-containing gas and/or hydrogen can beintroduced at one or more points on the reactor.

A preferred catalyst which selectively catalyzes the use of hydrogengenerally comprises oxides or phosphates selected from the groupconsisting of the oxides and phosphates of germanium, tin, lead,arsenic, antimony and bismuth.

A more preferred catalyst which catalyzes the combustion of hydrogencomprises at least one noble metal of transition group VIII of thePeriodic Table of the Elements. Examples of such catalysts aredescribed, for example, in the following documents:

-   -   U.S. Pat. No. 4,788,371, U.S. Pat. No. 4,886,928, U.S. Pat. No.        5,430,209, U.S. Pat. No. 5,530,171, U.S. Pat. No. 5,527,979 and        U.S. Pat. No. 5,563,314.

The dehydrogenation of propane is preferably carried out in the presenceof steam. The steam added serves as heat transfer medium and aids thegasification of organic deposits on the catalysts, thus counteringcarbonization of the catalysts and enabling the operating life of thecatalyst to be increased. The organic deposits are in this caseconverted into carbon monoxide and carbon dioxide.

For the purposes of the present invention, the dehydrogenated of propaneis preferably carried out in tray processes, with largely autothermaland therefore cost-effective operation being made possible in theextended integrated process for the synthesis of propylene oxide bycombustion of the hydrogen introduced via the substream T (6 a).

In the process of the invention, the quality of the propane feed is inprinciple not critical. The propane used can be fresh or recycledpropane and may further comprise additional by-products which have nosignificant influence on the dehydrogenation process.

The propane dehydrogenation can also be carried out continuously orbatchwise.

In a preferred embodiment of the propane dehydrogenation in step (i) ofthe process of the present invention, the substream T (0) produced inthis step comprises at least propene, propane and hydrogen. Furthermore,T(0) can further comprise gases from the group consisting of N₂, H₂O,methane, ethane, ethylene, CO and CO₂, either individually or asmixtures of two or more gases from this group, as by-products.

In the process of the present invention, the ratio of propane to propenein substream T (0) is in the range from 0.1 to 10, preferably from 0.5to 5, particularly preferably from 1.0 to 2.0. The ratio of hydrogen topropene in substream T (0) is in the range from 0 to 1.5, preferablyfrom 0.3 to 1.3, particularly preferably about 1.1.

In a mode of operation using oxygen as cofeed, i.e. additionallyintroduced oxygen or circulated gas from step (iii), viz. the hydrogenperoxide synthesis (T (6) or T (6 a)), the hydrogen to propene ratio ispreferably from 0.4 to 2.0.

In principle, all or some of substream T (0) is transferred via suitablemeans known to those skilled in the art, for example lines in the formof pipes, to step (ii) or, in the case of the extended integratedprocess for the synthesis of propylene oxide, to step (b).

Furthermore, it is also possible for substream T (0) to be fed to aseparation apparatus in an intermediate step following step (i) or step(a). In this, any by-products which have been formed in the propanedehydrogenation and may be present in T (0) can be separated off.

Step (ii) or step (b) comprises the fractionation of substream T (0) togive at least one gaseous hydrogen-rich substream T (2) and a liquidsubstream T (1) comprising at least propene and propane.

The fractionation of the substream T (0) can, for the purposes of theinvention, in principle be carried out by all methods which are known tothose skilled in the art and are technically possible in the presentcase and using the apparatuses which are suitable for the respectivemethod. For example, the fractionation can be carried out by means ofthe apparatus described in DE 100 28 582.1.

Thus, the separation of T (1) from substream T (0) in step (ii) or step(b) can be carried out by bringing the preferably cooled substream T (0)into contact with a preferably hydrophobic organic solvent in which theconstituents propane and propene present in T (1) are preferentiallyabsorbed.

Subsequent desorption, rectification and/or possibly stripping with aninert gas and/or an oxygen-containing gas, but for the purposes of thepresent invention preferably molecular oxygen, allows at least theconstituents propane and propene present in T (1) to be recovered.

The substream T (2) which comprises at least hydrogen represents thetailgas from the absorption. The absorption can be carried out either incolumns or in rotary absorbers. These can be operated in cocurrent or incountercurrent. Suitable absorption columns are, for example, traycolumns, columns containing structured packing and columns containingrandom packing. Of course, trickle and sprayed towers, granite blockabsorbers, surface absorbers such as thick film absorbers and thin filmabsorbers and also rotary columns, plate scrubbers, crossed sprayscrubbers and rotary scrubbers are also possible.

In principle, all absorption media which are known to those skilled inthe art and appear suitable for this purpose can be used. For thepurposes of the present invention, preference is given to usingrelatively nonpolar organic solvents which preferably have no externallyacting polar groups, e.g. aliphatic (e.g. C₈-C₁₈-alkanes), also aromatichydrocarbons such as middle oil fractions from paraffin distillation, orethers having bulky groups on the oxygen atom. Mixtures of two or moreof the solvents mentioned are also useful. Further solvents or solventmixtures which can be used as absorption media in the process of thepresent invention are listed in DE 100 28 582.1.

A solvent mixture which is preferably used as absorption medium for thepurposes of the present invention comprises biphenyl and diphenyl ether,preferably having the azeotropic composition, in particular a mixture ofabout 25% by weight of biphenyl and about 75% by weight of diphenylether (Diphyl®).

However, for the purposes of the present invention, substream T (0) ispreferably fractionated by condensation to give the substreams T (1) andT (2).

Thus, in the process of the present invention, the C₃ components of thesubstream T (0) can be wholly or partly condensed by, for example, useof heat exchangers, for example surface condensers or condensers withdirect or indirect air cooling.

Preference is given to using one or more shell- and-tube heat exchangersin the process of the present invention. In this case, cooling withinthe heat exchangers can be carried out using either air, water oranother suitable medium.

Thus, the substream T (0) can be wholly or partly condensed in step (ii)or step (b). Preference is given to virtually complete condensation ofthe C₃ components such as propene and propane present in the substream T(0).

In the process of the present invention, more than 90%, preferably morethan 95%, particularly preferably more than 99%, of the C₃ componentspresent in T (0) are separated off as substream T (1) by means of theabovementioned fractionation methods in step (ii) or step (b).

Substream T (1), which comprises at least the C₃ components propene andpropane, is passed via suitable lines known to those skilled in the artto step (iv) or step (c).

The gaseous substream T (2) which remains comprises hydrogen as maincomponent and possibly also a variable proportion of C₃ components andpossibly further gaseous, low-boiling components. The proportion of C₃components present in T (2) can be controlled via the conditions in step(ii) or step (b). The gas phase in question is conveyed as substream T(2) via lines known to those skilled in the art to step (iii). In thenovel extended process for the synthesis of propylene oxide, T (2) iscombined with a substream T (5 a) and conveyed via lines known to thoseskilled in the art to step (e).

In the integrated process for the synthesis of propylene oxide, thegaseous substream T (2) from step (ii) can have a ratio of hydrogen toC₃ components in the range of at least 90-95:10-5, preferably at least99:1 and particularly preferably at least 99.9:0.1.

The liquid substream T (1) which is produced in step (ii) or step (b)and comprises at least propene and propane is fractionated in a furtherstep (iv) or step (c) to give at least one propane-rich substream T (5)and at least one propene-rich substream T (3).

The fractionation of the substream T (1) can be carried out by allmethods which are known to those skilled in the art for this purpose,but is preferably carried out by thermal methods such as distillationand/or rectification.

In principle, all distillation processes suitable for the fractionationof the substream T (1) to give a propane-rich substream and apropene-rich substream can be used.

The fractionation unit used for this purpose in the process of thepresent invention basically comprises all constituent parts which areknown to those skilled in the art and are necessary to separate mixturesby fractional distillation into at least one propane-rich fraction andat least one propene-rich fraction. However, for the purposes of thepresent invention, the substream T (1) is preferably fractionated byrectification to give a propane-rich substream and a propene-richsubstream. In this process, the enrichment or separation of the liquidmixture occurs essentially by mass transfer between vapor and boilingliquid flowing in countercurrent. The rectification is carried out inone or more rectification columns which consist essentially of tubularseparation columns and also vaporizers and a condenser at the upper end(top) of the respective column.

In the process of the present invention, more than 80%, preferably atleast 90%, particularly preferably at least 95%, of the propene presentin T(1) is separated off in the substream T (3) in step (iv) or step(c). This propene-rich substream T (3) is passed to the further step (v)or step (g) via suitable lines.

In the process of the present invention, the propane-rich substream T(5) from step (iv) can be recirculated to step (i).

It is also possible for the propane-rich substream T (5) to be worked upby further methods known to those skilled in the art before it is fedinto step (i), e.g. it can be enriched in the propane present thereinbefore being fed into step (i).

Accordingly, the present invention also provides a process of theabove-described type in which the propane-rich substream T (5) is fed tostep (i).

In the extended integrated process for the synthesis of propylene oxide,the propane-rich substream T (5) produced in step (c) is transferred viasuitable lines to step (d).

In step (d), the substream T (5) is separated into at least thesubstreams T (5 a) and T (5 b). In a preferred embodiment of the presentinvention, the substream T (5) is purified to separate off by-productspresent in addition to propane before it is divided. This purificationcan be carried out by all methods known to those skilled in the art forthis purpose, e.g. distillation or absorption processes.

The separation of the substream T (5) is carried out by all methodsavailable to those skilled in the art for this purpose, for example bydivision by means of a multiway valve. Separation with simultaneouspurification, for example by means of distillation, is also possible.

Basically, in step (d), T (5) is separated into two substreams T (5 a)and T (5 b) which may comprise identical or different amounts ofpropane.

Substream T (5 b) is, according to the present invention, recirculatedto step (a) via lines known to those skilled in the art.

Substream T (5 a) can, if appropriate, be purified again and is thencombined with the hydrogen-containing substream T (2) from step (b) andtransferred to step (e).

Step (iii) or step (e) of the process of the present invention comprisesthe synthesis of hydrogen peroxide.

In step (iii) the hydrogen-containing substream T (2) is reacted withintroduced oxygen (X in FIG. 1), for example in the form of air, givinga substream T (4) which is rich in hydrogen peroxide and a gaseoussubstream T (6).

In step (e) of the extended integrated process for the synthesis ofpropylene oxide, the propane-containing substream T (5 a), which hasbeen combined with the hydrogen-containing substream T (2) with anintroduced oxygen-containing gas, e.g. air or molecular oxygen (X inFIG. 2) to give a substream T (4) which is rich in hydrogen peroxide anda gaseous substream T (6 a).

In a preferred embodiment of the extended integrated process, oxygen isintroduced in place of air. This avoids the accumulation and theassociated purging of any interfering gases.

If desired, not only oxygen or air but also hydrogen can be introducedinto step (iii) or step (e) by means of a suitable facility (Z in FIG. 1and FIG. 2).

Essentially all processes for synthesizing hydrogen peroxide which areknown to those skilled in the art can be used in step (iii) or step (e).

In a preferred embodiment of the processes of the present invention,hydrogen peroxide is prepared by direct synthesis from the elements. Forthe purposes of the present invention, it is possible to use all methodsfor the direct synthesis of hydrogen peroxide from the elements whichare known by those skilled in the art.

In the extended integrated process for the synthesis of propylene oxide,the use of the combined substreams T (2) and T (5 a) in step (e) offersthe advantage that it achieves a reduction in the amount of oxygen inthe reaction mixture of the hydrogen peroxide synthesis to below thelower explosive limit (4% H₂ to 96% of O₂), thus increasing the safetyof the process. The propane added via T (5 a) is thus a safe gas bufferfor the direct synthesis of hydrogen peroxide in this embodiment of theinvention.

A further associated advantage is that an increase in the hydrogenconcentration to a ratio of oxygen to hydrogen of 1 to 1 or above ismade possible in this way. Since the space-time yield of the directsynthesis of hydrogen peroxide increases in proportion to the hydrogenconcentration, this embodiment of the invention makes it possible tomake the reactor dimensions smaller and thus also decrease the operatingcosts.

In step (e) of the novel extended process for the synthesis of propyleneoxide, the amount of oxygen and hydrogen in the feed is adjusted so thatthe ratio in the substream T (6 a) from this step is optimal for use inthe propane dehydrogenation of step (a).

Accordingly, in the novel extended process for the synthesis ofpropylene oxide, the substream T (6 a) from step (e) is recirculated tostep (a) and thus makes it possible for the direct synthesis of propaneto be operated autothermally, as described above.

This mode of operation thus makes it possible to dispense withcirculated gas in the direct synthesis of hydrogen peroxide.

An example of a possible method of preparing hydrogen peroxide in theprocess of the present invention is the procedure disclosed in U.S. Pat.No. 4,009,252, in which hydrogen peroxide is formed from hydrogen andoxygen over palladium-containing catalysts. This reaction is carried outbatchwise. WO 92/04277, too, describes a process which can be used forthe purposes of the present invention and comprises reacting hydrogenwith oxygen in a tube reactor charged with an aqueous catalystsuspension to give hydrogen peroxide. A further possible way ofpreparing hydrogen peroxide for the purposes of the present invention isthe continuous process for preparing hydrogen peroxide which isdescribed in U.S. Pat. No. 5,500,202 and EP-A 0 579 109 and comprisesreacting H₂O₂ gas mixtures over a stationary, pulverulent catalyst in atrickle bed reactor.

A further process known from the prior art for preparing hydrogenperoxide which can be used for the purposes of the present invention isdescribed in U.S. Pat. No. 4,336,238 and U.S. Pat. No. 4,336,239. Inthis process, the reaction and oxygen to form hydrogen peroxide iscarried out over palladium-containing catalysts in organic solvents orsolvent mixtures which may also comprise water. U.S. Pat. No. 4,389,390describes a similar process in which the catalyst which has been leachedfrom the support is recovered by means of activated carbon filters.

However, the process for preparing hydrogen peroxide solutions which hasbeen developed by the applicant himself and is described in EP-A 0 946409 is particularly preferably used for the purposes of the presentinvention. This process allows the safe preparation of hydrogen peroxidesolutions having a hydrogen peroxide content of at least 2.5% by weight.In this process, hydrogen and oxygen are reacted continuously overcatalysts comprising palladium as active component, with the reactionbeing carried out over shaped catalyst bodies in water and/orC₁-C₃-alkanols as reaction medium. The shaped catalyst bodies arepreferably ordered catalyst packing (monoliths) and/or beds, or shapedbodies made up of meshes, for example metal meshes. In this preferredprocess, it is possible to use oxygen in the form of air.

In this process, the reaction is generally carried out in a floodedreactor. Water and/or C₁-C₃-alkanols, preferably water and/or methanol,serve as reaction medium. The reaction gas, which may comprise not onlyhydrogen and oxygen but also inert gases such as nitrogen or noblegases, generally has an O₂:H₂ ratio in the range from 1:100 to 100:1. Itis possible to circulate the reaction gas. Reaction gas and reactionmedium can be conveyed in cocurrent or in countercurrent relative to oneanother, preferably in cocurrent, with the liquid phase forming thecontinuous phase and the reaction gas forming the discontinuous phase.Preference is given to a vertical reactor construction (upright reactor)in which the reaction gas and reaction medium are preferably passedthrough the reactor in cocurrent from the bottom upward. Hydrogen can beintroduced into the reactor via one or more intermediate feed pointsdownstream of the feed point for the oxygen or air. The two-phase outputfrom the reactor can be taken off at the upper end of the reactor andseparated in a suitable separation vessel to give a substream T (4)which is rich in hydrogen peroxide and a gaseous substream T (6) or T (6a).

For the purposes of the present invention, the synthesis of hydrogenperoxide in step (iii) is carried out so that even when hydrogen/oxygenmixtures above the explosive range (O₂:H₂>20:1) are used,hydrogen-peroxide solutions having an H₂O₂ content above 2.5% by weightare obtained.

The substream T (4) which is rich in hydrogen peroxide comprises atleast hydrogen peroxide and water. Substream T (4) may further comprisehalides, acids, alcohols and further organic components and alsosensitizers and promoters for the hydrogen peroxide synthesis, e.g. CO.

If appropriate, T (4) can be worked up further by methods known to thoseskilled in the art.

Accordingly, the invention also provides a process of theabove-described type in which substream T (4) comprises at leasthydrogen peroxide and water.

All or some of the gas phase T(6) can be burnt, either after compressionin a suitable compressor or else directly, in a further step (vi) togenerate energy and the energy can be utilized in the step (i).

Accordingly, the present invention also provides a process as describedabove in which all or some of the gaseous substream T (6) from step(iii), which comprises a mixture of hydrogen and oxygen, is burnt in afurther step (vi) to generate energy and the energy is utilized in step(i).

This variant is particularly preferred when the substream T (6) fromstep (iii) comprises less than 4% of hydrogen in oxygen or less than 4%of oxygen in hydrogen.

The energy produced in step (vi) can be used in step (i) for heating theapparatuses used for the dehydrogenation of propane and/or forregeneration of the catalyst or catalysts used in the propanedehydrogenation.

Accordingly, the present invention also provides a process as describedabove in which the energy is utilized in step (i) for the followingpurposes, either individually or in combination with one another:

-   -   (aa) heating the apparatuses used in the dehydrogenation of        propane;    -   (bb) regeneration of the catalyst or catalysts used in the        propane dehydrogenation.

In a further embodiment of the present invention, substream T (6) can berecirculated either wholly or partly to step (iii).

To counter accumulation of inert compounds which may occur in the caseof complete recirculation of T (6), a substream of T (6) is transferredfrom time to time or continuously during the process for utilization instep (i). This procedure is preferably chosen whenever the gaseoussubstream T (6) contains more than 0.5%, preferably more than 0.7%,particularly preferably more than 1%, of a mixture of hydrogen andoxygen.

Accordingly, the present invention also provides a process of theabovementioned type in which all or some of the gaseous substream T (6)from step (iii) which contains more than 1% of a mixture of hydrogen andoxygen is recirculated to step (iii).

In a further embodiment of the integrated process for the synthesis ofpropylene oxide, all or some of the gaseous substream T (6) from step(iii) can be recirculated to step (i). The advantage of this procedureis that the gaseous substream T (6) which, in this embodiment, comprisesC₃ residues together with H₂ and O₂ in an H₂:O₂ ratio of from 1:100 to100:1 can be used for regenerating the catalyst used in step (i) fordehydrogenation of propane. The regeneration is carried out essentiallyby burning off all or some of the organic components deposited on thecatalyst surface.

Accordingly, the present invention also provides a process as describedabove in which all or part of the gaseous substream T (6) from step(iii) is recirculated to step (i).

The substream T (3) from step (iv) or step (c) is reacted in a furtherstep (v) or step (g) with the substream T (4) from step (ii) or step (e)to give propylene oxide.

However, the gas phase T (6 a) from step (e) of the process of thepresent invention can also be recirculated directly in a further step(f) to the step (a). In this case, the substream T (6 a) comprises thecomponents propane, oxygen and hydrogen in a molar ratio C₃:O₂:H₂ of1:0.01-1:0-2, preferably in a molar ratio C₃:O₂:H₂ of 1:0.03-0.3:0-0.6and particularly preferably in a molar ratio C₃:O₂:H₂ of1:0.04-0.2:0-0.4.

The reaction of the propane-rich substream T (3) with the substream T(4) which is rich in hydrogen peroxide to give propylene oxide can becarried out by means of all methods known to those skilled in the art.

For the purposes of the present invention, the reaction in step (v) orstep (g) is preferably the epoxidation of the propene from substream T(3) by means of hydrogen peroxide from substream T (4) in the presenceof a catalyst to form propylene oxide.

Possible methods of carrying out the epoxidation in question andpreferred epoxidation catalysts are described, inter alia, in DE 101 35296.4, DE 101 05 528.5, DE 100 32 884.9, DE 101 55 470.2, DE 101 37543.3 and DE 101 35 296.4.

Epoxidation catalysts which are particularly preferably used for thepurposes of the invention are Ti-zeolites having the MFI or MELstructure or an MFI/MEL mixed structure, Ti-containing zeolite catalystsdesignated as TS-1, TS-2, TS-3, and also Ti zeolites having a frameworkstructure isomorphous with β-zeolite.

Further details regarding the catalysts which can be used, in particularzeolites, may be found in, for example, DE 100 10 139.2, DE 197 23 950.1and DE 102 32 406.9 and the prior art cited therein.

Accordingly, the present invention also provides a process of theabovementioned type in which the reaction in step (v) is the epoxidationof the propene from substream T (3) by means of hydrogen peroxide fromsubstream T (4) in the presence of a catalyst to give propylene oxide.

In the process of the present invention, the degree of conversion intopropylene oxide is at least 80%, preferably at least 85%, particularlyat least preferably 95%.

The propylene oxide can be separated off from the mixture formed in thereaction in step (v) or step (g) by all methods known to those skilledin the art and can, if appropriate, be worked up further. Separationmethods and work-up processes which are preferred for the purposes ofthe invention are described in DE 198 35 907.1 and DE 100 01 401.1.

The mixture obtained in addition to propylene oxide in step (v) or step(g) can be recirculated wholly or partly as substream T (7) comprisingat least propane and propene to step (i) or step (a).

Accordingly, the present invention also provides a process of theabove-described type in which all or part of a substream T (7)comprising at least propane and propene and having a ratio of propane topropene of less than 1 which comes from step (v) or step (g) isrecirculated, if desired after a further work-up step, to step (i) orstep (a), or else is recirculated directly to step (iv) or step (c).

The work-up step in question can be carried out by methods known tothose skilled in the art, for example distillation, rectification ormembrane separation.

Tables 1 and 2 below illustrate possible embodiments of the process ofthe present invention, with Table 1 relating to the integrated processwhich is shown schematically in FIG. 1 and Table 2 relating to theextended integrated process which is shown schematically in FIG. 2.

LIST OF REFERENCE NUMERALS

FIG. 1

Step (i) dehydrogenation of propane

Step (ii) condensation

Step (iii) synthesis of hydrogen peroxide

Step (iv) fractionation

Step (v) synthesis of propylene oxide

Step (vi) combustion

T(0), T(1), T(2), T(3) substreams

T(4), T(5), T(6), T(7)

X air or oxygen

Y propylene oxide

Z hydrogen

FIG. 2

Step (a) dehydrogenation of propane

Step (b) condensation

Step (c) fractionation

Step (d) separation

Step (e) synthesis of hydrogen peroxide

Step (f) recirculation

Step (g) synthesis of propylene oxide

T(0), T(1), T(2), T(3) substreams

T(4), T(5), T(5 a), T(5 b),

T(6 a), T(7)

X oxygen

Y propylene oxide

Z hydrogen

1-12. (canceled)
 13. An integrated process for the synthesis ofpropylene oxide, comprising the following steps: (a) dehydrogenation ofpropane to give a substream T (0) comprising propane, propene andhydrogen; (b) fractionation of the substream T (0) to give at least onegaseous hydrogen-rich substream T (2) and a substream T (1) comprisingpropene and propane; (c) fractionation of the substream T (1) to give atleast one propane-rich substream T (5) and at least one propene-richsubstream T (3); (d) separation of the substream T (5) into at least thesubstreams T (5 a) and T (5 b); (e) synthesis of hydrogen peroxide usingthe substream T (2) which is combined with at least the substream T (5a), giving a substream T (4) which is rich in hydrogen peroxide and agaseous substream T (6 a); (f) at least partial recirculation of thesubstream T (6 a) to step (a); (g) reaction of the at least onesubstream T (3) with substream T (4) to give propylene oxide.
 14. Theprocess as claimed in claim 13, wherein the propane-rich substream T (5b) is fed to step (a).
 15. The process as claimed in claim 13, whereinsubstream T (4) comprises hydrogen peroxide and water.
 16. The processas claimed in claim 13, wherein the reaction in step (g) is theepoxidation of the propene from substream T (3) by means of hydrogenperoxide from substream T (4) in the presence of a catalyst to givepropylene oxide.
 17. The process as claimed in claim 13, wherein asubstream T (7) comprising propane and/or propene is obtained from step(g) and is wholly or partly recirculated to step (a).
 18. The process asclaimed in claim 17, wherein the propane-rich substream T (5 b) is fedto step (a), wherein a substream T (7) comprising propane and propeneand having a ratio of propane to propene of less than 1 is obtained fromstep (g) and is, if appropriate after a further work-up step, wholly orpartly recirculated to step (c).
 19. The process as claimed in claim 13,wherein a substream T (7) comprising propane and propene and having aratio of propane to propene of less than 1 is obtained from step (g) andis, if appropriate after a further work-up step, wholly or partlyrecirculated to step (c).
 20. An integrated process for the synthesis ofpropylene oxide, comprising the following steps: (a) dehydrogenation ofpropane to give a substream T (0) comprising propane, propene andhydrogen; (b) fractionation of the substream T (0) to give at least onegaseous hydrogen-rich substream T (2) and a substream T (1) comprisingpropene and propane; (c) fractionation of the substream T (1) to give atleast one propane-rich substream T (5) and at least one propene-richsubstream T (3); (d) separation of the substream T (5) into at least thesubstreams T (5 a) and T (5 b); (e) synthesis of hydrogen peroxide usingthe substream T (2) which is combined with at least the substream T (5a), giving a substream T (4) which is rich in hydrogen peroxide and agaseous substream T (6 a); (f) at least partial recirculation of thesubstream T (6 a) to step (a); (g) reaction of the at least onesubstream T (3) with substream T (4) to give propylene oxide, whereinthe propane-rich substream T (5 b) is fed to step (a) and wherein asubstream T (7) comprising propane and/or propene is obtained from step(g) and is wholly or partly recirculated to step (a).
 21. The process asclaimed in claim 20, wherein a substream T (7) comprising propane andpropene and having a ratio of propane to propene of less than 1 isobtained from step (g) and is, if appropriate after a further work-upstep, wholly or partly recirculated to step (c).
 22. The process asclaimed in claim 20, wherein substream T (4) comprises hydrogen peroxideand water and wherein the reaction in step (g) is the epoxidation of thepropene from substream T (3) by means of hydrogen peroxide fromsubstream T (4) in the presence of a catalyst to give propylene oxide.